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NATO forces confirmed in a statement that its planes fired Sunday on a group of vehicles that it believed contained insurgents who were about to attack its forces, only to discover later that women and children were in the cars.

NATO did not provide a figure of how many died or say if all those in the vehicles were civilians.

The Afghan government and NATO have launched an investigation.

Investigators on the ground have collected 21 bodies and two people are missing, Interior Ministry spokesman Zemeri Bashary said. Fourteen people were wounded, he said.

The strike hit three minibuses that were driving down a major road in the mountainous province. There were 42 people in the vehicles, all civilians, Bashary said.

NATO said that its forces transported injured people to nearby medical centers.

“We are extremely saddened by the tragic loss of innocent lives,” NATO commander Gen. Stanley McChrystal said in the statement. “I have made it clear to our forces that we are here to protect the Afghan people and inadvertently killing or injuring civilians undermines their trust and confidence in our mission. We will redouble our effort to regain that trust.”

McChrystal apologized to President Hamid Karzai for the incident on Sunday, NATO said.

NATO has gone to great lengths in recent months to reduce civilian casualties as part of a new strategy to focus on protecting the Afghan people to win their loyalty over from the Taliban. Rules for airstrikes have been tightened, but mistakes continue to happen.

In the continuing offensive against a Taliban stronghold in Helmand province, south of Uruzgan, two NATO rockets killed 12 civilians and others have gotten caught in the crossfire.